1. Field of the Invention
This invention relates to the preparation of homopolymers and copolymers of vinyl halide such as vinyl chloride having reduced grain size and melt viscosity, improved impact strength and easy processabiltiy. processability. polymers are useful in the production of films, coatings, and molded articles. Scale build-up is eliminated during bulk polymerization of the polymers of the invention.
The polymers of the invention have impact strength improved by as much as a factor of 10 over the unmodified prior art polymers. Additionally, the polymers of the invention have improved clarity and can be more easily processed to produce molded articles, since the polymers of the invention have excellent mold release properties allowing molds to be made using the polymer of the invention without the usual additional lubricant components of the use of mold release agents which are used on the mold itself in order to provide a surface to which the polymer will not stick readily.
Fine grain homopolymers and copolymers obtained by the process of the invention have narrow grain size distribution and 10 to 50 micron average grain size range, thus permitting the polymers of the invention to be used in applications where previously bulk polymerized polymers were unsuited because of their excessive grain size and wide grain size distribution. In prior art bulk polymerization methods, polymers are produced having average grain sizes in the range of about 80 to about 150 microns. More costly suspension polymerization processes are presently utilized to produce fine grain size polymers. The normal suspension processes produce grain sizes in the range of 100 to 200 microns.
The polymers of the invention have the further advantage that they are not contaminated with the suspending agents of suspension produced polymers.
2. Description of the Prior Art
Polyvinyl chloride has been produced by bulk or mass polymerization methods in the absence of solvents and diluents. The polymer produced has an irregular shape which may vary from minute globular particles to irregular, non-gobular masses. In British Pat. No. 1,047,489 and U.S. Pat. No. 3,522,227, both of which are hereby incorporated by reference, there are described a method of polymerization in mass in two steps which produces an improvement in regularity of granular size and shape. The process is more controllable and the product more uniform than can be obtained by other methods of bulk polymerization. In this process, the monomer is polymerized in a first stage to about 7 to about 15 percent conversion, preferably about 8 to about 10 percent conversion in a reactor in which provision is made for high speed, high shear agitation. At the end of the first stage polymerization, the mass is transferred to a second reactor and further polymerized using slow speed agitation to an appropriate end point (70 to 85 percent conversion in many cases). It is an object of this invention to produce polymers having small particle size which remain substantially unsolvated when placed in contact with primary type plasticizers for vinyl chloride resins.
Vinyl halide polymers of the prior art have been prepared in a wide variety of molecular weights; those of high molecular weight generally having better physical properties such as hardness and strength than those of lower molecular weight. The higher the molecular weight, however, the more difficult it is to process the polymer into a final product form.
In the subsequent processing of vinyl halide polymers, such as milling and extruding, higher molecular weight polymers require the use of higher shear force and/or higher temperatures to fluidize the polymer in comparison to lower molecular weight materials. The increased shear force and/or increased temperature required increases the internal temperature of the polymer and tends to degrade the vinyl halide polymers. In addition, the increased shear force necessary to process the polymers adds to the cost of processing the polymer. It is an object of this invention to produce easy processing polymers having improved clarity, greater fluidity at elevated temperatures reduced resistance to shear force without sacrificing other physical properties, greater stability against degradation on milling and molding and excellent mold release properties. Conventional lubricants and mold releasing agents such as polyethylene wax, silicone resin, fluorinated resins, etc., which are added to resins of the prior art to molding, can be eliminated when the polymers of the invention are used.
Mechanical blends of olefin polymers and polyvinyl chloride have been prepared in an effort to produce easy processing polyvinyl halide polymers. The mixtures are incompatible and unsatisfactory. Graft copolymers of polyethylene and vinyl chloride have been prepared as described in Belgian Pat. Nos. 657,762 and 652,069 using methods of suspension polymerization in an effort to increase the flexibility at low temperatures of polyvinyl halide polymers and copolymers as well as improve their impact resistance. These graft copolymers overcome the incompatibility of physical blends of polyolefins and polyvinyl chloride but these methods of producing graft copolymers are expensive and time consuming. It is an object of the present invention to produce graft copolymers by the bulk polymerization of an olefin-acrylic ester copolymer dissolved in a vinyl chloride monomer. The process of the invention provides graft copolymers having unexpectedly large improvements in impact resistance for the amount of olefin-acrylic ester copolymer used.
French Pat. No. 1,588,381 discloses a process for the single stage bulk polymerization of vinyl chloride in combination with an auxiliary polymer such as styrene-butyl acrylate-acrylic acid terpolymer to overcome the tendency toward the formation of curds and deposits on polymerization equipment which requires thereafter complicated cleaning operations. It is an object of the present invention to produce a high impact strength, improved processing, small particle size vinyl halide polymers by a two-stage bulk polymerization process in which vinyl chloride is graft polymerized with an olefin-acrylic ester copolymer.
U.S. Pat. No. 3,261,788 discloses that stable dispersions of synthetic polymers in an inert organic liquid can be made without using conventional stabilizing agents. Instead, stabilizing solvatable groups are used which become indirectly attached to the dispersed polymer particles through groups which themselves are merely adsorbed onto the surface of the dispersed particles. The term "solvatable group" means that the group is solvated by the inert organic liquid in which the polymer is dispersed. An example of a polymerization system having a solvatable group incorporated into the dispersed particle is a system wherein the polymer precipitates out of an inert organic liquid in the presence of a block or graft copolymer, of which one constituent is solvated by the inert organic liquid and the other constituent is compatible with and is coprecipitated with the polymer. The coprecipitation results in the compatible constituent becoming inextricably entangled with the precipitated polymer chains and hence becoming an integral part of the particle. The block or graft copolymer may be added as such or may be formed in situ during the formation of the polymer to be dispersed.